Hologram recording method, hologram recording reproduction method, hologram recording device, hologram recording/reproduction device and hologram reproduction device

ABSTRACT

In the hologram recording and hologram-record reproduction using a one-dimensional optical modulator, the one-dimensional optical modulator where a plurality of light-modulating pixels are arranged modulates a laser beam to record a digital-data signal on a hologram recording medium, and part of the light-modulating pixels record sync signals in two or more positions on the hologram recording medium with retaining a predetermined interval, thereby improving hindrance to reproduction or readout of information recorded on the hologram recording medium, a decrease in S/N, and so on due to deformation or the like of the hologram recording medium.

TECHNICAL FIELD

The present invention relates to a hologram recording method, ahologram-record reproducing method, a hologram recording apparatus, ahologram recording and reproducing apparatus, and a hologram reproducingapparatus.

BACKGROUND ART

The basic structure for hologram recording is as follows; a laser beamfrom a laser beam source is split into two beams, one of the split laserbeams is modulated by an optical modulator in accordance with recordinginformation to be signal light, the other of the split laser beams ismade to be reference light, both the light beams are superimposed on ahologram recording medium, and interference fringes due to a change inthe refractive index caused by the interference of both the laser beamsare formed on the hologram recording medium, whereby information isrecorded.

Reproduction of information from the hologram recording is performed asfollows; the signal light is cut, only the reference light is applied tothe hologram recording medium with the same position and angle ofincidence as those when recording hologram, the interference fringes areformed on the recording medium, whereby reproduced light based ondiffracted light corresponding to the original signal light can beobtained, and accordingly the reproduced light is detected by a sensorsuch as CCD (Charge Coupled Device) (refer to, for example, HolographicData Storage by H. J. Coufal, D. Psaltis, G. T. Sincerbox p. 350,Springer Series Verlag, July 2000, Optical Science, and Optical DataStorage 2001, Proceedings of SPIE Vol. 4342 (2002) p. 567).

The hologram recording as described above is performed as follows; usinga one-dimensional optical modulator in which a plurality of, forexample, 1088 light-modulating pixels are arranged in one dimension,that is, for example, using GLV (Grating Light Valve) (refer to GratingValve Technology: Update and Novel applications, for example), signallight is obtained based on a light and dark (white and black) imageoptically modulated from a laser beam according to information of “1”,“0” from each light-modulating pixel, and the signal light is applied tothe hologram recording medium together with the reference light toperform hologram recording. In this case, when reproducing the record,reproduced light from the hologram recording medium is detected by aone-dimensional photodetector having a plurality of light-detectingportions where light-detecting elements of appropriate size are arrangedat appropriate positions corresponding to the light-modulating pixels inan optical modulator.

However, in this case, when the position and angle of the hologramrecording medium at the time of recording is shifted at the time ofreproduction, the position of the reproduced image also shifts and sothere may be some cases where the intended reproduction becomesdifficult.

FIG. 11 schematically shows a relation of arrangement between theone-dimensional optical modulator 101 having a plurality oflight-modulating pixels P1, P2, P3 . . . Pn and the one-dimensionalphotodetector 102, in which the number n of detecting elements D1, D2,D3, . . . Dn in the one-dimensional photodetector 102 is selected to bethe same, for example, 1088 as that of the light-modulating pixels inthe one-dimensional optical modulator 101.

However, according to the above structure, when signals “1”, “0”modulated by an optical modulator 1 are digital data supplied as ON/OFFof light for example, in the case where the one-dimensional opticalmodulator 101 performs modulation in such a manner that, for example,signal “1” is given to alternate pixels and signal “0” is given to theother alternate pixels among the light-modulating pixels P1, P2, P3 . .. Pn, if the shift occurs between the light-modulating pixels P (P1, P2,P3, . . . Pn) in the one-dimensional modulator 101 and the detectingelements D (D1, D2, D3 . . . Dn) in the one-dimensional photodetector102 by, for example, one-half, “1” and “0” enter detecting elements halfby half, as a result, no light and dark (white and black) image can beobtained but, for example, a uniformly gray one is obtained, which meansthat detection of information is impossible.

The occurrence of such shift is quite common. Specifically, because thehologram recording medium often takes the form of a disc or card and therecording and reproduction is repeated frequently, when the hologramrecording medium is installed in the hologram recording apparatus orreproducing apparatus, a shift in position and a shift in angle are verylikely to occur.

Moreover, the hologram recording medium is often made of organicmaterials and thus often requires ultraviolet-ray irradiation and heattreatment during recording or after recording; on this occasion, several% of shrinkage is often caused. Therefore, not only the above-describedsimple shift occurs in the reproduced position, but also one-to-onecorrespondence between the light-modulating pixels P in theone-dimensional optical modulator 101 and the detecting elements D inthe one-dimensional photodetector 102 will collapse. Such phenomena alsomake the reproduction difficult or decrease the S/N.

DISCLOSURE OF THE INVENTION

The present invention is to provide a hologram recording method,hologram-record reproducing method, hologram recording apparatus,hologram recording and reproducing apparatus, hologram-recordreproducing apparatus capable of efficiently avoiding theabove-described obstacle to hologram-record reproduction, a decrease ofS/N and so on.

Specifically, a hologram recording method according to the presentinvention includes the steps of: modulating a laser beam by aone-dimensional optical modulator in which a plurality oflight-modulating pixels are arranged and recording a digital-data signalon a hologram recording medium, and recording sync signals on thehologram recording medium in two or more positions with retaining apredetermined interval by means of part of light-modulating pixels inthe one-dimensional optical modulator.

A hologram reproducing method according to the present inventionincludes the steps of: applying reference light to the hologramrecording medium having a digital-data recording section and sync-signalrecording sections formed in two or more positions with a predeterminedinterval, reading the digital data and the sync signals, and detectingthe shift in the position of the digital-data signal by the syncsignals.

Further, a hologram recording apparatus according to the presentinvention includes: a laser-beam source, split means for splitting alaser beam from the laser-beam source into a first laser beam and secondlaser beam, a one-dimensional optical modulator in which a plurality oflight-modulating pixels are arranged, and a hologram-recording-mediumdisposing portion; in which part of the light-modulating pixels in theone-dimensional optical modulator optically modulates part of the firstlaser beam by a digital-data signal, at least part of the otherlight-modulating pixels in two or more positions optically modulates atleast part of the other of the first laser beam by a sync signal toobtain signal light of the laser beam, and the signal light andreference light of the second laser beam are applied to the hologramrecording medium to form a recording section for the digital-data signaland recording sections for the sync signals in two or more positionswith retaining a predetermined interval.

A recording and reproducing apparatus according to the present inventionincludes a laser-beam source, split means for splitting a laser beamfrom the laser-beam source into a first laser beam and second laserbeam, a one-dimensional optical modulator in which a plurality oflight-modulating pixels are arranged, a hologram-recording-mediumdisposing portion, and a photodetector at least in one-dimension havinga larger number of light-detecting elements than that oflight-modulating pixels in the one-dimensional optical modulator; inwhich at the time of recording, part of the light-modulating pixelsoptically modulates part of the first laser beam by a digital-datasignal and at least part of the other light-modulating pixels in two ormore positions in the one-dimensional optical modulator opticallymodulates at least part of the other of the first laser beam by syncsignals to obtain signal light of the laser beam; the signal light andreference light of the second laser beam are applied to the hologramrecording medium to form a recording section for the digital-data signaland recording sections for the sync signals in two or more positionswith retaining a predetermined interval; and

at the time of reproduction, the reference light of the second laserbeam is applied to the hologram recording medium to supply reproducedlight obtained from the hologram recording medium to the light-detectingelements in the photodetector for detecting the digital-data signal andsync signals to detect a shift in a position of the digital-data signalby the sync signals.

Further, in the above-described recording apparatus and further, forexample, in the above-described recording and reproducing apparatusaccording to the present invention, the one-dimensional opticalmodulator has an array of a plurality of reflecting ribbons, and eachreflecting ribbon may have a diffraction-grating structure in which aphase of an arrived laser beam is modulated and reflected correspondingto the displacement thereof and diffracted light is generated.

Further, a hologram reproducing apparatus according to the presentinvention includes a laser-beam source, a photodetector in which aplurality of light-detecting elements are arranged, and a disposalportion for disposing a hologram recording medium having a recordingsection for a digital-data signal and recording sections for syncsignals in two or more positions, in which the hologram recording mediumdisposed in the disposal portion is irradiated with reference light of alaser beam from the laser-beam source to obtain reproduced lightincluding the digital-data signal and sync signals recorded on thehologram recording medium, and the reproduced light is detected by thephotodetector to detect a shift in a position of the digital-data signalby the sync signals.

As described above, in the present invention not only a digital-datasignal, namely, original recording information signal but also syncsignals are recorded on a hologram recording medium, and inreproduction, the original information signal as well as the syncsignals are detected so as to detect a shift in a position of theoriginal information signal.

Since the detection of a position can be made in this way, it ispossible to correct a shift of a position and a shift of an angle in ahologram recording apparatus, hologram reproducing apparatus or the likedue to repeated use of the hologram recording medium, and further tocorrect a shift in a position of a recording signal caused by shrinkageand so on of the hologram recording medium.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic constitutional diagram of an example of a hologramrecording and reproducing apparatus according to the present invention;

FIG. 2 is a schematic plan view of an example of a one-dimensionaloptical modulator in the apparatus according to the present invention.

FIG. 3 is a perspective view of one pixel in an example of aone-dimensional optical modulator in the apparatus according to thepresent invention;

FIG. 4 is a sectional view of one pixel in an example of aone-dimensional optical modulator according to the present invention;

FIGS. 5A and FIG. 5B are sectional views each showing an operating stateof an example of a one-dimensional optical modulator;

FIG. 6 is a diagram showing a relation between pixels in aone-dimensional optical modulator and detecting elements in aone-dimensional photodetector in an example of the apparatus accordingto the present invention;

FIG. 7 is a schematic constitutional diagram of an example of thehologram reproducing apparatus according to the present invention;

FIG. 8 is an explanatory view showing a corresponding relation betweenmodulation by a one-dimensional optical modulator and light-receivingintensity of a one-dimensional photodetector in an example of thehologram recording and reproducing method according to the presentinvention;

FIG. 9 is a diagram showing light-receiving intensity of light-detectingelements (pixels) in CCD as a one-dimensional photodetector;

FIG. 10 is a diagram showing a relation between light-receivingintensity of light-detecting elements (pixels) in CCD as theone-dimensional photodetector in FIG. 9 and digitized values “1” and “0”thereof; and

FIG. 11 is a diagram showing a relation between a one-dimensionaloptical modulator and a one-dimensional photodetector in a conventionalhologram recording and reproducing apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described.

First, an embodiment of a hologram recording apparatus according to thepresent invention will be described with reference to the schematicconstitutional diagram of a recording and reproducing apparatus inFIG. 1. However, the present invention is not limited to thisembodiment.

[Hologram Recording Apparatus]

In the embodiment shown in FIG. 1, a light-transmissive type hologramrecording medium is employed as a hologram recording medium 1.

Recording on the hologram recording medium 1 is performed as follows; adiameter of a laser beam L from a laser-beam source 10 is expanded by abeam expander 11 in a Y-direction perpendicular to the paper plane andis introduced into a half-mirror 12 which splits the laser beam into afirst laser beam L1 and second laser beam L2 having the optical paths ina Z-direction and in a X-direction, respectively, which areperpendicular to the Y-direction and to each other.

The first laser beam L1 is condensed in the Z-direction by a cylindricallens 13 and introduced into a one-dimensional optical modulator 14.

The first laser beam L1 is hereupon optically modulated into a signallight, which is condensed by a first lens 21 and passes through apinhole 15H.

A second lens system 22 condenses the laser beam L1, that is, a signallight onto a hologram recording medium 1.

On the other hand, the second laser beam L2 split by the half-mirror 12in the X-direction is condensed as reference light by a third lenssystem 23 onto the hologram recording medium 1. In this way, a hologramrecording is performed on the hologram recording medium 1 by aninterference between the first laser beam (signal light) and the secondlaser beam (reference light).

Next, an embodiment of a hologram recording and reproducing apparatuswill be described with reference to FIG. 1.

[Hologram Recording and Reproducing Apparatus]

This embodiment includes a hologram-recording-apparatus section 2 and ahologram-reproducing apparatus section 3.

In the hologram-recording-apparatus section 2, recording is made ontothe hologram recording medium 1 by the same structure and operation asdescribed above.

The hologram-reproducing apparatus section 3 includes a fourth lenssystem 24 and a one-dimensional photodetector 16.

This hologram-reproducing apparatus section 3 can be formed, forexample, using part of the structure of the hologram-recording-apparatussection 2 in common. However, in this case, the first laser beam L1 willbe cut in reproducing operation.

The hologram-reproducing apparatus section 3 in this case includes thelaser-beam source 10, beam expander 11, half-mirror 12 and the thirdlens system 23 in optical system relating to the second laser beam L2 asreference light, and further the fourth lens system 24 and theone-dimensional photodetector 16 disposed behind the hologram recordingmedium 1.

By applying the reference light to the hologram recording medium 1 inthe same condition of incidence as in recording, reproduced light Ls canbe extracted from the hologram recording medium 1 as if it weretransmitted light of the signal light L1.

The fourth lens system 24 described above is disposed at a diffusingposition on an optical path of the reproduced light Ls, so that thereproduced light Ls is condensed onto the one-dimensional photodetector16. In this case, an optical image by the reproduced light Ls in theone-dimensional photodetector 16 is obtained as a light and dark imagecorresponding to the light and dark image obtained in theone-dimensional optical modulator 14.

In the above-described hologram recording apparatus and hologramrecording and reproducing apparatus, it is desirable for theone-dimensional optical modulator 14 to employ a one-dimensional GLV(Grating Light Valve) array having, for example, an electrostaticdriving type structure made of a reflective type diffraction controlgrating with high modulating efficiency and quick response.

As is shown in FIG. 2, the GLV includes a large number, for example,1088 pieces of pixels 30 arranged in a line, each of which has adiffraction grating structure and forms one light-modulating pixel.

As is shown in a perspective view of FIG. 3 and in a sectional view ofFIG. 4, each pixel 30 has, for example, six ribbons 32 on a substrate 31made of silicon substrate, for example; each ribbon 32 is supported atboth ends, reflects the laser beam L1 and is arranged in parallel toform a diffraction grating.

As is shown in a cross-sectional view of FIG. 5A, these ribbons 32 arearranged on one plane, where the center portion of each ribbon retains arequired distance from the surface of the substrate 31 by supportportions at both ends.

As is shown in FIG. 4, the ribbon 32 includes an electrode layer 34 madeof a metal film constituting a reflective surface, which is formed on aninsulation film 33 made of, for example, silicon nitride.

On the other hand, a common opposing electrode 35 is formed on thesubstrate 31, which extends across under the arranged ribbons 32 and isopposite to the electrode layer 34 of each of ribbons 32 so as to retaina required gap with the ribbons 32.

In this structure, by applying a required voltage between the electrodelayers 34 of alternate ribbons 32 and the opposing electrode 35, thesealternate ribbons 32 are displaced toward the opposing electrode 35, asshown in FIG. 5B, by an amount equivalent to λ/4 where λ is a wavelengthof the laser beam L1 applied to the GLV.

By doing in this way, when the above laser beam L1 is applied to thepixels 30 in a line, each constituting a light-modulating pixel, in thestate in which the six ribbons 32 corresponding to one light-modulatingpixel are situated on the same one plane as shown in FIG. 5A, the pixel30 as the light-modulating pixel does not operate as a diffractiongrating; and the laser beam L1 arrived at the light-modulating pixel ismerely reflected, and with respect to this light-modulating pixel, forexample, information of “1” as signal light of “light” is recorded onthe hologram recording medium 1.

In contrast, when a displacement of λ/4 is caused by applying a requiredvoltage for a signal of, for example, “0” between the electrode layer 34of alternate ribbons 32 and the opposing electrode 35 to modulate aphase of laser beam arrived here and to make the laser beam reflect,laser beams reflected at adjacent ribbons 32 interfere with each otherto cause substantially no reflected light, whereby the laser beam issubstantially lost with respect to this pixel, that is, light-modulatingpixel and is modulated into a signal of “dark” of “0”; on this occasion,for example, information of “0” is recorded on the hologram recordingmedium 1.

When the one-dimensional optical modulator 14 having the diffractiongrating structure is employed in this manner, higher order light than ±first order light and ± second order light of diffracted light will begenerated, however, such light is cut off by the above-described shield15.

The ribbon 32 of each pixel, that is, light-modulating pixel 30 can beselected to be, for example, 3 μm in width, 100 μm in length, and 100 nmin thickness; and the distance between the ribbons 32 and the opposingelectrode 35 is selected to be, for example, 650 nm. A pitch of pixels30 can be made to be 25 μm.

The one-dimensional photodetector has an array in which a plurality oflight-detecting elements are arranged. The one-dimensional photodetectorcan be formed of, for example, CCD (Charge Coupled Device) whoselight-receiving portion is arranged in one dimension. The number ofthese light-detecting elements or the arrays is selected to be largerthan the number of pixels in the one-dimensional optical modulator 1.

In other words, as shown in FIG. 6, when n pieces of light-modulatingpixels P, that is, P1, P2, P3 . . . Pn are arranged in theone-dimensional optical modulator 14, the number N of thelight-detecting elements in the one-dimensional photodetector 16, namelythe light-receiving portions D1, D2, D3, . . . DN, for example, in thecase of CCD is made to be n<N.

Further, the one-dimensional optical modulator can also be formed ofpart of a two-dimensional optical modulator, for example.

The optical systems, for example, the first to fourth optical systems 21to 24 can be selected so that an optical image on the one-dimensionalphotodetector 16 may correspond to the optical image in theone-dimensional optical modulator 14.

[Hologram Reproducing Apparatus]

FIG. 7 shows an embodiment of a hologram reproducing apparatus accordingto the present invention. This hologram reproducing apparatus has thesame structure as that of the hologram-reproducing apparatus section 3in the hologram recording and reproducing apparatus described in FIG. 1.In FIG. 7, the parts corresponding to those in FIG. 1 are denoted by thesame symbols and not described repeatedly.

Next, an embodiment of the recording method according to the presentinvention will be described.

[Hologram Recording Method]

In this embodiment the hologram recording is performed using thehologram recording and reproducing apparatus according to the presentinvention shown in FIG. 1.

In this hologram recording method, of all pixels P in theone-dimensional optical modulator 14 having a plurality oflight-modulating pixels P, light-modulating pixels in two or morepositions retaining a predetermined interval are employed aslight-modulating pixels for sync signals and the laser beam L1 isoptically modulated by the sync signals. The other major part of pixelsP is used to optically modulate the laser beam L1 arrived at theone-dimensional optical modulator 14 based on the digital-data signal oforiginal recording information.

The modulation using the sync signals is performed in such a patternthat is not used in the modulation method based on the digital-datasignal. For example, when the 8-10 modulation method is performed, “1”or “0” continues from 1T to 4T, but does not continue on and after 5T.Thus, when this modulation method is performed, for example, “1” in 6Tis defined as a sync signal.

Next, an embodiment of the hologram reproducing method will bedescribed.

[Hologram Reproducing Method]

In this embodiment, for example, using the hologram recording andreproducing apparatus shown in FIG. 1, the first laser beam L1 is cutand only the second laser beam L2 as reference light is applied to thehologram recording medium 1, or using the hologram reproducing apparatusshown in FIG. 7, the second laser beam L2 is applied to the hologramrecording medium retaining the same position and angle of incidence andso on as those in recording. When doing so, the reproduced light Lscorresponding to the signal light L1 is obtained. In other words, alight and dark image corresponding to the light and dark image obtainedby the one-dimensional optical modulator 14 can be obtained on theone-dimensional photodetector 16. Therefore, the digital-data signal andsync signals can be detected by the one-dimensional photodetector.

Further, since these sync signals are disposed in two positionsretaining a predetermined interval, these sync signals are used asposition-detecting signals to correct a shift in a position of thedigital data, and therefore the digital data obtained from theone-dimensional photodetector 16 can be detected correctly.

Next, a practice example of the recording and reproduction according tothe present invention will be described.

[Practice Example]

In this example, the recording and reproduction was performed using thehologram recording and reproducing apparatus whose schematicconstitution is shown in FIG. 1 in accordance with the above-describedprocedure. In this case, the 8-10 modulation method was employed as themodulation method for a digital-data signal. In this method, asdescribed above, the pattern of “1” and “0” signal continues until 4T,but does not continues on and after 5T. Accordingly, the pattern of syncsignal was defined as “1” in 6T.

The sync signal pattern was disposed at both ends of an array oflight-modulating pixels in the one-dimensional optical modulator 14,that is, a pair of sync signals was disposed.

Further, as described above, this practice example employed the opticalsystem for making the image in the one-dimensional optical modulator 14at the time of recording correspond one to one with the image ofreceived light in the one-dimensional photodetector 16 at the time ofreproduction.

The one-dimensional optical modulator 14 employed the above-describedGLV and the one-dimensional photodetector 16 employed the CCD array.

The GLV had a structure in which 1088 modulating pixels P were arrangedwith a 25 μm pitch, and the one-dimensional photodetector 16 had astructure in which light-detecting elements, that is, light-receivingportions D were arranged with a 7 μm pitch. Therefore, in this case, thenumber of portions D became 25/7=3.6 per pixel P. Although the result isnot integral number, sync signals are disposed with a predeterminedinterval in the method according to the present invention and theposition of a digital-data-signal section is detected by a relativeposition of the sync signals, so that no problem occurs.

FIG. 8 schematically shows the image in the one-dimensional opticalmodulator 14 at the time of recording and the image of received light inthe one-dimensional photodetector 16 at the time of reproduction in thispractice example.

This was the case where the above-described “1” in 6T as sync signal wasdisposed at both ends. Specifically, the sync signal was disposed in thefirst to sixth pixels P1 to P6 from one end and in pixels Pn to Pn-5 atthe other end, respectively.

In this way, information recorded on the hologram recording medium isreproduced. In other words, by detecting the pair of sync signals, aninterval between them can be known. Based on this information, acorrection signal can be obtained, which corrects a shift in aninformation signal, that is, digital-data signal if a shift in positionor a shift in angle occurs, or shrinkage and so on is caused in thehologram recording medium itself, when the hologram recording medium 1is disposed at the hologram-disposed portion of hologram recording andreproducing apparatus in recording and reproduction, as described at thebeginning.

It should be noted that at the time of hologram reproduction, in thelight-receiving portion (light-detecting element) D of the CCD at aposition corresponding to, for example, a boundary between “1” of“light” and “0” of “dark”, a gray portion where the light is mixed withthe dark will be generated. FIG. 9 shows the intensity of light in thevertical axis, which is received by an array of sixty CCDlight-receiving portions (horizontal axis), for example, at No.0 toNo.60 in the CCD array; and in this case, an intermediate “gray”intensity between “1” and “0” may be detected at a positioncorresponding to the boundary between “1” of “white” and “0” of “black”.

However, because the CCD can usually detect 28 tones, the intensity oflight at the boundary position can be detected correctly, and byperforming signal processing based on this output, as shown in FIG. 10,it is possible to make them into binary values of digital “1” and “0”with an intermediate light intensity 0.5 between “1” and “0” as adividing position.

Therefore, in the above-described practice example, it is also possibleto divide the detected gray portions at a detecting element D in theone-dimensional photodetector 16, corresponding to the boundary betweenpixels P6 and P7 and so on into binary values.

Note that although the 8-10 modulation method has been described as anexample in the above-described embodiment, the present invention isapplicable to other various modulation methods, and in those othermodulation methods, the sync signals are also made in a pattern unusedin this modulation method. These sync signals may be disposed at bothends of the one-dimensional optical modulator 14 as in theabove-described embodiment or may not be disposed at both ends, providedthat they are disposed at positions retaining a predetermined interval.

Having described the case where the GLV having an excellent response toa laser beam and a high modulating efficiency is employed as theone-dimensional optical modulator 14 in the above-described embodiment,the DMD (Digital Micromirror Device), liquid crystal, and so on can alsobe employed; and thus, the present invention is not limited to theabove-described embodiment, practice example, and so on and variousmodifications and alterations can be made therein.

As described above, according to the present invention, sync signals arerecorded on a hologram recording medium together with a digital-datasignal, that is, an original recording information signal, and inreproduction, the original information signal as well as the syncsignals are detected, thereby enabling a position of the informationsignal, that is, digital-data signal to be detected by the sync signals.

Since the position detection is performed in this way, it is possible tocorrect the shift in a position and the shift in an angle in thehologram recording apparatus, hologram reproducing apparatus or the likedue to repeated use of the hologram recording medium, and further tocorrect the shift in a position of recorded signal due to shrinkage andso on of the hologram recording medium.

Moreover, because the digital data can be detected securely even thoughthe hologram recording medium is made of organic materials, or even ifthe shrinkage occurs when applying ultraviolet-rays during recording orafter recording, or in heat treatment, it is possible to avoiddifficulty in the reproduction and a decrease in S/N due to collapse ofthe corresponding relationship between the light-modulating pixels P inthe one-dimensional optical modulator 1 and the corresponding detectingelements D in detector.

1. A hologram recording method comprising the steps of: modulating alaser beam by a one-dimensional optical modulator in which a pluralityof light-modulating pixels are arranged; recording a digital-data signalon a hologram recording medium; and recording sync signals on saidhologram recording medium in two or more positions retaining apredetermined interval by means of part of the light-modulating pixelsin said one-dimensional optical modulator.
 2. A hologram-recordreproducing method comprising the steps of: applying reference light toa hologram recording medium having a digital data recording section, anda sync signal recording section formed in two or more positionsretaining a predetermined interval, reading said digital data and saidsync signals, and detecting a shift in a position of said digital-datasignal by the sync signals.
 3. A hologram recording apparatuscomprising: a laser beam source, split means for splitting a laser beamfrom the laser beam source into a first laser beam and a second laserbeam, a one-dimensional optical modulator in which a plurality oflight-modulating pixels are arranged, and a hologram-recording-mediumdisposing portion, wherein part of the light-modulating pixels in saidone-dimensional optical modulator optically modulate part of said firstlaser beam by a digital-data signal, and at least part of the otherlight-modulating pixels in two or more positions in said one-dimensionaloptical modulator optically modulate at least part of the other of saidfirst laser beam by sync signals to obtain signal light by a laser beam,the signal light and reference light by said second laser beam areapplied to a hologram recording medium to form on the hologram recordingmedium a recording section for said digital-data signal and recordingsections for said sync signals in two or more positions having apredetermined interval.
 4. A hologram recording apparatus according toclaim 3, wherein said one-dimensional optical modulator is formed of anarray of a plurality of reflecting ribbons, and each reflecting ribbonhas a diffraction grating structure in which a phase of an arrivinglaser beam is modulated by the displacement of the ribbon to bereflected and diffracted light is generated.
 5. A hologram recording andreproducing apparatus comprising: a laser beam source, split means forsplitting a laser beam from the laser beam source into a first laserbeam and a second laser beam, a one-dimensional optical modulator inwhich a plurality of light-modulating pixels are arranged, ahologram-recording-medium disposing portion, and a photodetector atleast in one dimension, having a larger number of light-detectingelements than that of the light-modulating pixels in saidone-dimensional optical modulator, wherein at the time of recording,part of light-modulating pixels in said one-dimensional opticalmodulator optically modulate part of said first laser beam by adigital-data signal and at least part of the other light-modulatingpixels in two or more positions in said one-dimensional opticalmodulator optically modulate at least part of the other of said firstlaser beam by sync signals to obtain signal light; the signal light andreference light by said second laser beam are applied to the hologramrecording medium to form on the hologram recording medium a recordingsection for said digital-data signal and recording sections for saidsync signals in two or more positions retaining a predeterminedinterval; and at the time of reproduction, the reference light by saidsecond laser beam is applied to said hologram recording medium, saidlight-detecting elements in said photodetector receive reproduced lightobtained from the hologram recording medium, and said digital-datasignal and said sync signals are detected to detect a shift in aposition of said digital-data signal by the sync signals.
 6. A hologramreproducing apparatus comprising: a laser beam source, a photodetectorincluding an array of a plurality of light-detecting elements, and ahologram-recording-medium disposing portion having a recording sectionfor digital-data signal, and recording sections for sync signals in twoor more posit ions, wherein reference light by a laser beam from saidlaser beam source is applied to the hologram recording medium disposedin said disposing portion to obtain reproduced light including saiddigital-data signal and said sync signals recorded on the hologramrecording medium, and the reproduced light is detected by saidphotodetector to detect a shift in a position of said digital-datasignal by said sync signals.